The present application relates to portable computers and their speaker systems.
Background: Computer-Based Audio
The production of quality sound with computers has advanced significantly in recent years. Early personal computers typically had nothing more than a single, small speaker used to produce a beep or series of single-frequency beeps to indicate system status upon startup. However, consumers demanded more. With the significant advances in circuit miniaturization, improved sound quality electronics has been available for desktop systems for some time, and is now moving into the portable computer systems arena.
Today, there is a multimedia explosion that is adding a significant new dimension to the way people use computers. One of the biggest advantages is 16-bit, CD-quality sound. With the right speakers, this sound can be as impressive as the sound on a normal consumer's stereo system. More and more, computer applications include the capability to use and capture the full impact of 16-bit sound for multimedia. MIDI musical compositions become more inspiring, games more suspenseful, software more dramatic, and radio and television programs (obtained through the computer) can equal the quality of conventional reception systems.
In addition, computer and audio technologies are revolutionizing business. For example, in video conferencing, voices are reproduced with a natural quality that makes them seem as if the speaker were in the room. Providing a high-quality audio background during presentations greatly enhances their effectiveness, and portable computers provide the greatest opportunity for meeting this need. Furthermore, the work environment is becoming ever more mobile with employees traveling more, and perhaps even telecommuting (working from home). Use of increased travel time translates to work environments with higher ambient noise, such as airports, trains, or mass transit systems. Another factor in this trend is the user's need for more effective methods for communicating with potential customers, or even with the office when away on business.
Background: Spatial Impression
The design of sound reproduction systems is not only based on considerations of electrical and acoustical engineering and physics, but also requires knowledge of psychoacoustics, i.e. how sound is perceived by listeners.
One of the parameters of psychoacoustics is spatial impression. When a sound is generated in a room, the listener will first hear the sound via the direct path from the source. Shortly thereafter, the listener will hear the reflections of the sound off surfaces such as walls or ceilings. Human listeners will assess the size of the space they are in by listening to laterally reflected sound which accompanies a sound signal. Thus, in a loudspeaker system it is desirable to have some sound transmission paths which reach the ears of the listener with a certain amount of delay (e.g. 10-60 milliseconds) as compared with the direct transmission path. (This will give the impression of a spacious listening room by broadening the soundstage, and also by giving the illusion of pushing the sound beyond the physical location of the speakers, even if the listener is listening in a room which is very small.) Moreover, another peculiarity of this psychoacoustic parameter is that the low frequencies (below 500 Hz) dominate spatial impression. A more roomy spatial impression is welcome to many listeners. However, it is not easy to do this with a small speaker system, and particularly not with a small portable computer's speaker system.
To increase spatial impression, the ratio of laterally reflected energy to directly transmitted energy can be increased: the higher this ratio, the greater the spatial impression. (An extreme case of this is found in a symphony concert hall, where there is almost no direct sound.)
Whether the user is video-conferencing, playing a game, or just working with music in the background, spatial impression plays an important role in the computing experience. That role is growing ever more important as multimedia makes its way into even the most uncompromising of business applications.
Further background regarding spatial impression can be found in: J. Blauert, SPATIAL HEARING (2.ed. 1996); and in M. Barron, "Effects of Early Reflections on Subjective Acoustic Quality in Concert Halls" (thesis, University of Southampton, 1974); both of which are hereby incorporated by reference.
Background: Stereophonic and Surround Sound
Since its introduction in the 1950's, stereo has been regarded as an essential minimum requirement of quality sound reproduction. Stereo can convey a traditional soundstage in which the sound comes from the front, such as when one attends a play or concert. However, even stereo has shortcomings when required to convey the ambiance where the sound is heard from all around the listener. Stereo's lack of spatiality undermines sonic realism in, for example, a game where aircraft fly overhead from front to back, or footsteps come from off to the side. For this reason various "surround sound" schemes have been used, to provide at least some speaker output behind the listeners' positions.
Background: Portable Computer Audio Limitations
Two driving constraints on the development of portable computers are volume and power consumption. The form factor of portable computers cannot be expanded, so that volume is a key resource. Additional functions are continually sought to be added, and many of these place new demands on available volume. Moreover, as larger display sizes become available they require more area in the lid, and hence consume more volume. Similarly, users continually demand more battery lifetime, which implies a continuous struggle to minimize power consumption.
These two constraints bind especially tightly in the case of speakers: the low-frequency power-efficiency of a speaker is directly affected by its volume. Since low-frequency response is a significant limitation of portable speaker systems, this is a difficult constraint. Some attempts have been made to use external speakers with portable computers, but this is cumbersome.
As of 1997, the internal sound systems typically sold with portable computers produce marginal sound quality at best. Most multimedia computers sold today include a stereo sound card which is capable of reasonably good sound reproduction. However, the internal speakers are typically small and produce tinny sound with inadequate bass. As a result, millions of multimedia computer users suffer very poor sound fidelity from their computers.
However, poor sound quality in portable computers is undesirable. The experience of a game or multimedia presentation, with sound and video animation, can be astonishingly real when coupled with quality audio. Quality sound can make a significant difference in the enjoyment the user can get from a multimedia computer. Music while working is more enjoyable, training and "edutainment" titles are richer, and games become much more realistic and exciting.
Background: Dynamic Speakers
One simple way to generate the air motion in a speaker is with a dynamic transducer. This is basically a very simple linear motor, in which a moving diaphragm is attached to a coil which is driven by a variable current. The coil is suspended in a constant magnetic field. The current through the coil interacts with the magnetic field to generate a force, which makes the coil and diaphragm oscillate according to the current variations through the coil.
Background: Piezoelectric Speakers
A piezoelectric material is one which changes shape in response to an applied electric field. Piezoelectric ("piezo") speakers use piezoelectric materials to generate physical motion in response to applied voltage. This physical motion typically has high force available, but operates over only a very short distance, so this is (mechanically) a high-impedance transducer. Thus the piezoelectric element is usually connected to a plate of some sort, and the motion of the plate provides most of the moving area of the speaker.
Such speakers typically are thin polymer sheets (of a piezoelectric polymer such as "PVDF") or disks of a piezoelectric ceramic. Such speakers can be made very thin; unlike diaphragm-type conventional speakers, piezo speakers need essentially no room for magnet and voice coil excursion requirements. More recently, ACX (Active Control Experts, Inc.) has manufactured packaged piezo speakers which take into consideration the problems encountered when working with piezo technology. These problems include breakage, attachment of leads, and electrical and mechanical isolation. The packaged model may now comprise polyimide coatings, pre-attached electrical leads, structural protection for the brittle piezo ceramics, and protection to the user (from high voltages associated with the use of piezos) as well as to the speaker from harsh working environments.
The convenient small form factor of piezo speakers is especially convenient for mounting in the lid of a portable computer. In such a configuration, the plastic back surface of the lid provides the moving area of the transducer. Thus the volume of the lid can be substantially dedicated to the display screen itself, and the piezo speakers can be mounted somewhere out of view.
However, piezo speakers are typically used only for midrange and high frequencies. This is because the low-frequency range of piezo speakers is typically limited by excursion limitations. (That is, the speaker plate moves in and out only a short distance.)
Enhancement of Audio Spatial Impression in a Portable Computer Using Dynamic and Piezoelectric Transducers
The present application discloses new techniques for implementing a system with four (or more) speakers, using dynamic and piezoelectric ("piezo") transducers. (Piezo devices are typically high frequency in nature.) If unequalized, these piezo drivers will roll off around 400-500 Hz. However, with equalization they can provide a good output power down to about 250 Hz (depending on the size of the baffle).
A particular advantage of piezoelectric speakers is that they can be placed wherever desired. For example, the middle of the back of the display lid of a portable computer is a particularly advantageous place to locate piezoelectric drivers. However, the speakers may be moved to any location in the back of the display to provide the desired effect. The high dynamic forces of piezo speakers will actually move the surface of the panel lid. Of the various modes of the panel lid, it is of course preferable to drive the modes which are "air-pumping" modes, i.e., which provide good acoustic emission, as compared to non-air-pumping.
The advantages of this piezoelectric configuration include:
1. greater front-back image depth; PA1 2. added spatial enhancement; PA1 3. a surround sound effect from the multiple echoes; and PA1 4. overall system constraints are significantly reduced due to the thickness of the piezo.